It is known to drive a track-laying vehicle with a drive system of the type described in U.S. Pat. No. 3,938,604. With this system, the main drive shaft that is normally continuously rotated by a heavy-duty diesel engine is connected through a fixed-ratio transmission to a pair of output shafts each connected to one input of a respective summing transmission having another input and having an output connected to the respective right or left track. The other inputs of these summing transmissions are connected through appropriate gearing to a so-called compensating shaft, with the gearing set-up so that if the compensating shaft is turned in one direction, one of the inputs is rotated in one direction and the other in the opposite direction and vice versa. Thus, if the compensating shaft is driven in one direction the output of the one summing transmission will rotate more slowly and the output of the other summing transmission more rapidly, and vice versa.
A variable-ratio hydrostatic transmission has an input connected to the main drive shaft and an output connected through an appropriate transmission to the compensating shaft. This hydrostatic transmission normally comprises a pump and a motor, with the pump normally being reversible and of variable volume. Thus, if the control element of this pump is left at the null or central point the compensating shaft is fixed and the outputs of both the summing transmissions will rotate at the same speed for straight-ahead travel of the vehicle. If tripped to one side of the central position the vehicle will veer to one side, and if the control element is tipped to the other side of the central position, the vehicle will veer to the opposite side.
This system works relatively well providing a stepless variation of the speeds of the two tracks for relatively sensitive steering control. Nonetheless the drive system is rather bulky. A four-element planetary-gear transmission is used to connect the output of the hydrostatic transmission to the compensating shaft, and is provided with three separate brakes. As a result of the considerable amount of mechanical parts the system is therefore relatively bulky and expensive to manufacture, and furthermore as a result of the large number of gears that are constantly turning in mesh with one another the losses in such a system are relatively high. What is more, it is difficult to conveniently achieve a relatively wide variation between the rotation rate of the drive gear for the one track relative to that for the other track for a very sharp turn, or even for turning in place.